The present invention relates to a process for removing metal contaminants from waste lubricating oil and in particular to the removal of metal contaminants by contacting the waste oil with an aqueous solution of salts to render the contaminants phase separable.
Waste lubricating oils include those oils which, having been used as lubricant or motor oils, contain impurities associated with their use. The major contaminants include metals such as zinc which is added to the oil with antiwear agents, and lead which, as a gasoline additive, contaminates the oil. Calcium is present in the oil arising from the addition of detergents. Iron accumulates in the oil as a result of wear on the engine parts. Since large volumes of these used oils exist, a better process is needed for removing the metal contaminants to provide a useful oil product which is suitable as a cheap fuel or lubricant.
It has previously been known to reprocess waste lubricating oil by contacting the oil with sulphuric acid, thereby forming an acid sludge with the contaminants, which is separated from the oil. This method suffers the disadvantage of low yields of refined oil, losing up to 20% of the original starting oil. Disposal of the acid sludge has also created environmental problems.
Other methods have been proposed for reprocessing waste oil by obtaining a methathesis reaction with the metal contaminants by adding an aqueous solution of reagents which precipitate the metals to render them phase separable from the oil. In accordance with these schemes, aqueous solutions of sodium silicate or sodium hydroxide have been used to precipitate the metals into an aqueous layer separable from the remaining oil phase. These reagents have not been found satisfactory in the past since the sodium ion has an emulsifying effect and forms powerful detergents with the organics--this keeps the oil and aqueous layers inseparable.
According to U.S. Pat. No. 3,930,988, issued to Johnson, used motor oils may be contacted with an aqueous solution of ammonium sulphate and/or ammonium bisulphate to precipitate the metal contaminants into an aqueous phase separable from the oil phase. This system requires highly acidic conditions to effectively remove the contaminants and is thus subject to corrosive problems associated with the low pH.